In comparison to a display using a cathode ray tube, a liquid crystal display element (LCD) consumes less electric power and is smaller and lighter. Systems in practical use that utilize such advantages include the twist nematic system, in which the twist angle between alignment layers of upper and lower substrates is 90 degrees; the super twist nematic system, in which the twist angle between alignment layers of upper and lower substrates is 180-260 degrees; and the in-plane switching system, in which the twist angle between alignment layers of upper and lower substrates is 0 degrees. In the twist nematic system and the in-plane switching system, liquid crystals are driven by the MIM system using a two-terminal thin layer transistor, and the TFT system using a three-terminal thin film transistor. (The MIM and TFT systems are collectively called the active matrix system.)
Properties required by liquid crystal compositions used in active matrix type display elements include:
(1) high specific resistance of the liquid crystal composition itself, and high voltage retention in the cell of the liquid crystal display element; PA1 (2) wide nematic-phase range of the liquid crystal composition itself; PA1 (3) the anisotropy in the refractive index of the liquid crystal composition can be changed adequately according to the cell thickness of the liquid crystal display element; and PA1 (4) the threshold voltage of the liquid crystal composition itself can be changed adequately according to the drive circuit of the liquid crystal display element.
In recent years, notebook-type portable personal computers which can be used outdoors have been developed, and the use of LCDs has been widespread. As compared with LCDs for indoor machines, LCDs for portable machines are restricted in driving power sources. In order that an LCD can be driven outdoors for a long time, the power consumption of the LCD must be decreased. In order that the power consumption of the LCD can be decreased, the threshold voltage of the liquid crystal composition itself must be decreased.
In view of such a background, Japanese Patent Application Laid-open No. 2-233626 discloses a trifluoro compound whose relatively large anisotropy of inductivity is suitable for a liquid crystal compound for active matrix type display elements. Application Example 2 of the above Patent Application discloses a composition comprising 15 percent by weight of a trifluoro compound and 85 percent by weight of a difluoro compound. However, the threshold voltage of the liquid crystal composition of this Example is high, making this composition unsuitable for fabricating portable LCDs.
WO94/03558 discloses a composition comprising trifluoro and difluoro compounds. In liquid crystal compositions disclosed in Examples 1 and 2 of the above patent, the upper temperature limit of the nematic liquid crystal phase is as low as 50.degree. C., and in Example 4 of the above patent, the threshold voltage is high. Therefore, this liquid crystal composition is not suitable for fabricating portable LCDs.
Although liquid crystal compositions have been intensively studied to meet various purposes, new improvement is always required.
An object of the present invention is to provide a liquid crystal composition which has in particular a low threshold voltage and a wide temperature range of the nematic phase, while maintaining various properties required for liquid crystal compositions for active matrix type display elements.